hot water tank as buffer with plate heater supply from wood boiler

archibald tuttle

ran into an interesting job that had a burnham hydronic 4 section from the late 80s that was replaced under warranty around 2003 and now leaking again. Around the same time as the warranty replacement a wood boiler was added with undersized feeds (about 120 feet of 3/4" pex with no diffusion barrier - ah the good ole days). At least there was a certain presence of mind, if not high awareness on the oxygen infiltration or btu capacity aspects, and the wood boiler supply is run into a stainless plate heat exchanger.

the system side of that exchanger is piped as a zone on the boiler (there were three zones and they combined one and took the third zone for the heat exchanger and that is just tricked to run on the boiler aquastat. And the boiler has a tankless supplying domestic hot water.

the boiler seldom starts in the winter according to oil usage, but i haven't observed operation long enough on a cold day to see if both heat legs are calling and domestic is turn on if and how quickly the boiler kicks in for keep up but i wouldn't be surprised.

so the second burnham is now leaking, haven't had the skin off to see if its a crack or between sections or . . . ( plenty of threads i'm sure to talk about this habit of those boilers) but at least I can't indict potentially oxygenated water from the uncoated leads to the wood boiler because of the plate heat exchanger. Owner is understandably perplexed at cast iron boilers that seem to have service life of condensing boiler and my suggestion is a change of strategy.

I'm thinking to put an indirect tank for the domestic hot water to also act as a buffer tank pulling and storing heat from the plate exchanger a bit like the boiler is now. this guy will eventually repipe with larger barrier leads from the wood boiler and maybe at that point it could be direct without the exchanger but for now I would use that loop for the domestic as a primary loop.

then i would secondary pipe the two heating feeds off the return to the exchanger to protect the domestic tank from seeing lower feed temps because return water from heating loops would cycle through the plate exchanger before hitting the tank again.

so this is adult employee usage of hot water. the biggest difficulty is that with the most heating buffer storage the DHW would be very hot and then at heavier heating loads you would see cooler domestic temps so it would need to be used judiciously or i would need a good variable tempering valve which i have often found is an oxymoron, at least insofar as one that is performing well a few years down the pike compare to new. recommendations?

I also could consider a good flow switch to kill the heating circulators on domestic demand (whatever takagi uses in their on demand heaters seems to be bombproof but i haven't had as much luck finding cost effective standalone, recommendations welcome) or I could use a two setpoint aquastat or two aquastats and set one as a lower limit that kills the heating pumps until the tank temp catches up which would be the equivalent of domestic priority - because i wouldn't want to set the primary as priority, as most of the time there is no problem running the heating loops while the buffer is making up.

finally, the backup function (wood boiler is tended all the time but if it got a leak or needed to be shut down for repairs) /summer DHW I recommended piping a small oil boiler as a secondary on the feed from the exchanger to the indirect DHW tank. as currently piped using the boiler as the buffer, if anything is wrong with the oil boiler or its controls the whole system is down.

So for that reason and as buffer for DHW use I kind of focused on the secondary piping with supply on the feed side and heating demand on the return as a little more predictable in utilizing the buffer and allows domestic primary from a piping standpoint and then using control as a backstop for that. I suppose I could just always run the DHW/system pump constant on when any heating load is on which is pretty much required anyway, i suppose I could use a variable pump for that function and then i'm less likley to get reverse circulation if i piped a more typical feed return piping across the loop. I don't need the secondary setup to avoid low temp return to boiler but it seemed maybe like a decent way to approach this.

please critique away, thanks,

brian

hot_rod

There are a handful of proven designs in this Idronics 10. Both piping and wiring schematics.

Decisions that need to be made on these duel fueled systems include storage, and how automated you want the controls to be. Always of course is the $$ budget

Personally I cannot always babysit my wood boiler. So I want the LP backup to roll in only as needed. It needs to be able to go off line when the wood becomes back up to speed. And under some conditions both boilers can contribute to the load, on design days for example.

You should not heat a large buffer with the oil fired boiler, if it is sized properly send it directly to the load.

Many DHW options, an indirect, heat exchanger on a current tank water heater. An instantaneous DHW via a plate HX.

Lack of return temperature protection function could be killing the cast boilers?

Siggys Heating with Renewable Energy textbook is invaluable for designing state of the art multi-fueled systems.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

archibald tuttle

thanks for that link hotrod. principle constraint here is that the feeds from the wood boiler are not sufficient size to manage transfer of available btus on an as needed basis at higher loads so that is why i'm viewing the indirect tank as a bit of a buffer placing 40 gallons of heated water in the building. but my piping plan is similar to those with heat exchangers with the that i'm planning to close the heat exchanger loop with the indirect boiler supply tappings as a 1" primary and then have the aux boiler and the heating zones piped as secondaries. I just like the idea of constantly moving as much heat as possible from the heat exchanger into storage. The key need i haven't filled yet is a good tempering valve for the DHW. Do you have a caleffi to recommend?

i don't think the auxilary/backup boiler is seeing low return temps against fire generally speaking. it seldom fires during heating season anyway and and is principally used to provide DHW in the off season. (originally it was the building heat before the wood boiler was installed and right now it is essentially serving the function of a buffer most of the time during heating season).

I haven't observed operation at 'design' conditions although the it has low limit aquastat so i believe it is called before a temperature drop into condensing range. I haven't fathomed the entire wiring scheme yet which is a hodgepodge because it was built up from the primary boiler set up with add ons before the era of multiple zone relays.

And i spent some time on the phone with your competition at honeywell the other day and their front line techs don't have a good grasp of low limit functions in their oil aquastat/relay combos so i pretty much try to watch cycles in cold weather to get an emperical idea of what the control is doing.

I still can't get straight myself amongst the alphabet soup of numbers which ones keep warm with no call. I've got a digital one on a job I was on last week which claims to be 'cold start' by model number, but it doesn't let the boiler drop below 145 even with no call and not a lot of differential on that low limit. And for reasons unknown (maybe because its an OEM part) the low limit and its differential and whether it is constant on or responding to call and just killing circulators until minimum temp is made are unaddressable as parameters, although the generic version of that control is supposed to provide installer access to those functions.

hot_rod

Well certainly that 3/4 piping will be the weak link. You can pump yourself out of most undersizing issues, but excessive velocity, noise and erosion will be the byproduct ion 10 fps velocity.

I guess you would need to define the loads to really crunch out the best options. 40 gallons of buffer could be depleted in 15 minutes if the loads are such. Wood buffers are usually 500 gallons and larger both for run time and drawdown capacity.

I'd probably suggest 1-1/4 lines to the wood stove before I touched a job like that. You will be a doing chasing it's tail trying to sort that out with that many problems. It could get expensive also.

We have a new 520 AngleMixer valve for DHW, also comes as a tank mixer kit, with a nice straight thru pattern it is a clean install.

archibald tuttle

@hot_rod

is that 520 temperature compensated or does it just mix a set volume of hot and cold?

hot_rod

archibald tuttle said:

@hot_rod

is that 520 temperature compensated or does it just mix a set volume of hot and cold?

All thermostatic mix valves should maintain the temperature you set, unless the hot supply falls below the temperature you set of course.

You want at least 20° warmer to the H port than the mixed temperature. So for 120 at the mixed port you want at least 140 at the H port.

Unique about the 520, if the cold fails the valve shuts down, a scald function similar to a point of use valve.

archibald tuttle

@hot_rod
got it. so as long as i'm maintaining at least 20 degrees hotter than the setpoint it is a temperature regulating valve so even if i have 150 degree or 160 degree feed water it would alter relative flows accordingly to maintain the 120. But if delivery drops below 140 I might experience cooler DHW.

thanks

hot_rod

archibald tuttle said:

@hot_rod
got it. so as long as i'm maintaining at least 20 degrees hotter than the setpoint it is a temperature regulating valve so even if i have 150 degree or 160 degree feed water it would alter relative flows accordingly to maintain the 120. But if delivery drops below 140 I might experience cooler DHW.

thanks

Yes, with any thermostatic mixing valve there needs to be some differential so the copper "motor" inside has some ability to move. When the hot drops too low the piston is at it's retracted and has no resolution.

archibald tuttle

@bebelynaccessdoors
Well it's all done and working anyway. I'm pretty happy with performance although there are a lot of ways up any mountain and only way I'd be 100% sure I took a pretty good one is to try a few others side by side, but glad to be able to do that rhetorically here since I usually. Not going to run several approaches experimentally on each job.